1. Field of the Invention
This invention relates in general to the field of recording media. In particular, one embodiment of this invention provides an erasable optical storage media and write/read/erase mechanism therefor in which data may be recorded and erased in response to thermal effects and, in particular in response to light.
2. Description of Related Art
Optical data storage media in the form of compact disks are well known as an alternative to longplaying records and magnetic tape cassettes. The disks with which consumers are familiar are optical read-only disks and the common disk player is designed specifically for this type of disk. These disks have a reflective surface containing pits which represent data in binary form. A description of these pits and how they function is provided by Watkinson, "The Art of Digital Audio," Focal Press, Chapter 13.
Compact disks are currently produced by a pressing process similar to the process used to produce conventional long playing records. The process, referred to herein as the "mastering" process, starts by first polishing a plain glass optical disk. This disk has an outside diameter from 200 to 240 mm, a thickness of 6 mm and undergoes various cleaning and washing steps. The disk is then coated with a thin chrome film or coupling agent, a step taken to produce adhesion between the glass disk and a layer of photo-resist, which is a photosensitive material. Data on a compact disk master tape are then transferred to the glass disk by a laser beam cutting method.
The glass disk is still completely flat after it is written on by the laser beam because pits are not formed until the glass is photographically developed. The disk surface is first made electrically conductive and then subjected to a nickel evaporation process. The disk, now known as the glass master, then undergoes nickel electrocasting, a process which is similar to that used in making analog phono records. A series of metal replications follow, resulting in a disk called a stamper. The stamper is equivalent to a photographic negative in the sense that it is a reverse of the final compact disk; that is, there are now bumps were there should be pits. This stamper is then used to make a pressing on a transparent polymer such as polyvinyl chloride, poly(ethyl-metacrylate) and polycarbonate. The stamped surface is then plated with a reflective film such as aluminum or other metal and finally a plastic coating is applied over the film to form a rigid structure.
The player operates by focusing a laser beam on the reflective metal through the substrate and then detecting reflected light. The optical properties of the substrate, such as its thickness and index of refraction, are thus critical to the player3 s detection systems and standard players are designed specifically with these parameters in mind.
The pits increase the optical path of the laser beam by an amount equivalent to a half wavelength, thereby producing destructive interference when combined with other (non-shifted) reflected beams. The presence of data thus takes the form of a drop in intensity of the reflected light. The detection system on a standard player is thus designed to require greater than 70% reflection when no destructive interference occurs and a modulation amplitude greater than 30% when data is present. These intensity limits, combined with the focusing parameters, set the criteria for the compact disks and other optical data storage media which can be read or played on such players.
Media on which data can be recorded directly on and read directly from have a different configuration and operate under a somewhat different principle. One example is described in U.S. Pat. No. 4,719,615 (Feyrer et. al.).
The medium disclosed in Feyrer et. al, includes a lower expansion layer of a rubbery material which expands when heated. The expansion layer is coupled to an upper retention layer which is glassy at ambient temperature and becomes rubbery when heated. Both layers are supported on a rigid substrate. The expansion and retention layers each contain dyes for absorption of light at different wavelengths. Data are recorded by heating the expansion layer by absorption of light from a laser beam at a "record" wavelength to cause the expansion layer to expand away from the substrate and form a protrusion or "bump" extending into the retention layer. While this is occurring, the retention layer rises in temperature above its glass transition temperature so that it can deform to accommodate the bump. The beam is then turned off and the retention layer cools quickly to its glassy state before the bump levels out, thereby fixing the bump. Reading or playback of the data is then achieved by a low intensity "read" beam which is focused on the partially reflecting interface between the retention layer and air. When the read beam encounters the bump, some of the reflected light is scattered, while other portions of the reflected light destructively interfere with reflected light from non-bump areas. The resulting drop in intensity is registered by the detector. Removal of the bump to erase the data is achieved by a second laser beam at an "erase" wavelength which is absorbed by the retention layer and not the expansion layer. This beam heats the retention layer alone to a rubbery state where its viscoelastic forces and those of the expansion layer return it to its original flat configuration. The write, read and erase beams all enter the medium on the retention layer side, passing through retention layer before reaching the expansion layer.
The erasable optical storage medium system described in Feyrer et. al., has a number of disadvantages. For example, the writing and erasure of data must be performed at two different wavelengths of light.
Further, the device relies on reflection at the interface between the retention layer and air which results in an inherently low reflectivity (30% maximum). Thus the system cannot be read by the detection mechanism of a standard compact disk player designed for focusing through a 1.2 mm polycarbonate substrate and requiring 70% reflectance. Still further there is either a predetermined level of thermal conductivity between the heated expansion layer, to sufficiently raise the temperature of the retention layer so that it can accommodate the bump formed by the expansion layer, or the retention layer must absorb a predetermined amount of light energy at the "record" wavelength, in order to produce the needed temperature rise in the retention layer during recording. In either case this requirement must be met and accurately controlled if this media is to be produced with consistent recording characteristics. In addition, in order for the most effective erasure to be achieved, the retention layer must be heated separately from the expansion layer. This follows from the fact that during erasure the retention layer must reach a rubbery state in order for the visoelastic forces of a cool expansion layer to pull the expansion layer back to its original flat configuration. If the expansion layer is heated during this time, it will not be in its relaxed state and it will therefore not return to its flat configuration. Since the expansion layer and the retention layers are in intimate physical contact, heat energy must be conducted between the two layers during both the recordation and erase processes, thus negating the possibility of only heating the retention layer. Any attempt to erase the medium during the act of recordation, i.e., direct overwrite data update, would therefore prove unsuccessful.